The present invention relates, in general, to methods of forming power semiconductor devices and, more particularly, to methods of forming insulated gate semiconductor devices.
Power semiconductor devices are used in such applications as variable speed motor controllers, uninterruptible power supplies, and high frequency welders. Included in this category of semiconductor devices are power metal oxide semiconductor field effect transistors (MOSFET's) and insulated gate bipolar transistors (IGBT's). Both types of devices offer similar gate drive and peak current capabilities as well as a wide safe operating area (SOA). However, IGBT's have superior conduction characteristics compared to power MOSFET's, whereas power MOSFET's are generally superior to IGBT's in terms of switching speeds.
Typically, IGBT's include a substrate layer of a P conductivity type on which a relatively lightly doped epitaxial layer of an N conductivity type is formed, thereby forming a PN junction. Most of the IGBT circuitry is fabricated in the epitaxial layer (more commonly referred to as the drift region), wherein the substrate layer serves as a bottom-side contact for the IGBT and forms an emitter region of a PNP transistor. The light doping of the epitaxial layer produces a drift region having a low conductivity, a high resistivity, and is capable of supporting high voltages. However, the high resistivity increases the "on" resistance, or the resistance during forward conduction, which in turn limits the current rating of the IGBT. The PN junction formed between the substrate and the epitaxial layer lowers the "on" resistance by injecting minority carriers into the drift region. In addition, the injection of minority carriers increases the conductivity of the drift region.
The modulation of the "on" resistance and the conductivity of the drift region by injection of minority carriers across the PN junction implies that both majority and minority carriers participate significantly in current flow in the IGBT. Although the use of both carrier types is advantageous for "on" resistance and conductivity, during shut-off of the IGBT the carriers produce a "tail" current which delays the shut-off of the device. It is believed that the charge associated with the "tail" current may be reduced and the shut-off delay improved by, for example, providing recombination centers in the lattice structure of the epitaxial layer. These recombination centers may be formed by creating imperfections or damage in the epitaxial layer lattice structure using such means as irradiating the epitaxial layer. Another solution is to insert a buffer zone between the substrate and the drift region, wherein the buffer zone is epitaxial silicon having the same conductivity type as the drift region but having a higher concentration of impurity material.
Accordingly, it would be advantageous to have a method of forming an IGBT having a lower "on" resistance, and a greater conductivity in the drift region. It would be further advantageous that the method exclude the formation of recombination centers and a buffer zone as these steps increase the cost and cycle time associated with manufacturing IGBT's, as well as the minimum thickness of wafers from which IGBT's are manufactured. In addition, the method should improve the switching speed of the IGBT.